Add textAlignment property to BTRPopUpButton

Two changes:

1. Add a `textAlignment` property to `BTRPopUpButton` that allows for layouts like this:

[Left Alignment](https://f.cloud.github.com/assets/353158/999124/f98a098a-0a21-11e3-9d82-44080772c115.png)
[Center Alignment](https://f.cloud.github.com/assets/353158/999126/268908b4-0a22-11e3-8f4a-6219e7881ea5.png)
[Right Alignment](https://f.cloud.github.com/assets/353158/999128/2cb6eb20-0a22-11e3-91bf-2191f7e526c4.png)
2. Also notable in the above screenshots, I've adjusted the code that positions the `NSMenu` to take into account situations where there's no image and position the menu to align with the label. This is also the reason why I'm simply not using the `alignment` property of `NSTextField`, because doing the layout manually allows accurate retrieval of the text frame for positioning the menu.